Lubricating oil for wet brake or wet clutch which is used in lubrication of parts including a wet brake or a wet clutch is required to be low in low temperature viscosity in view of starting performance. In general, the low temperature viscosity of lubricating oil can be easily decreased by decreasing the viscosity of the total base oil. In this case, however, the viscosity of the lubricating oil is too low at high temperatures, thereby producing a problem that the lubrication performance is decreased and the lubricating oil is unsuitable for practical use.
Therefore a method of compounding viscosity index improvers such as polymers to the low viscosity base oil has been widely used. This method, however, fails to solve the above problem because such polymers undergo viscosity reduction under shearing.
The first object of the present invention is to provide a base oil which holds a constant viscosity at high temperatures as one of the characteristics thereof and which is low in low temperature viscosity. It is, of course, required for the base oil to be excellent in oxidation stability and also in rubber swelling properties (seal rubber compatibility).
The second object of the present invention is to provide a lubricating oil composition in which friction characteristics for wet brakes or wet clutches are increased by the base oil itself.